In sealing devices used for hydraulic cylinders that are adapted to be used as driving means for construction machinery, there have been proposed a variety of types of sealing devices in order to improve the lubrication property of sliding surfaces (see under-mentioned individual patent documents).
For example, in a patent document 1, a sealing device is proposed which is constructed of a seal ring made of PTFE (polytetrafluoroethylene resin) and a thrust ring made of a rubbery elastic material is proposed. Although this sealing device is intended to reduce sliding resistance by the use of PTFE for the seal ring which slides with respect to a counterpart member, it is inferior, in pressure resisting ability against hydraulic pressure, to those made of nylon (polyamide type resin), so it would sometimes be damaged or broken upon application of high pressure while giving rise to a phenomenon of protrusion. In addition, a spiral groove formed on a sliding surface of the seal ring acts to place a pressure side and a non-pressure side in communication with each other to form an oil leakage passage, as a result of which there is a problem of low sealing performance.
In addition, in a patent document 2, there is proposed a sealing device which is intended to improve the lubrication property by providing a plurality of annular protrusions on a sliding surface of a seal ring, which serve to retain lubricant. However, there is the following problem. That is, even if a flaw or the like is generated in each annular protrusion at one place thereof, lubrication grooves between adjacent ones of the individual annular protrusions are placed in communication with each other, so that a leakage passage leading from a pressure side up to a non-pressure side is liable to be formed, thus easily reducing the sealing performance. In another words, it can be said that a safety factor with respect to the sealing performance is low.
Moreover, in a patent document 3, there is proposed a sealing device which is intended to improve the lubrication property by providing a plurality of holes which serve to connect between an inner peripheral surface and an outer peripheral surface of a low friction ring, but there is a problem that the strength of the low friction ring itself is decreased due to the provision of the holes. In addition, there is also another problem that upon application of high pressure, an elastic seal ring (a back ring made of rubber) is caused to protrude into the holes in the inner peripheral surface of the low friction ring, thus resulting in its damage.
Here, reference will be made to a sealing device that is proposed in a patent document 4, while referring to FIG. 12. FIG. 12 is a schematic cross sectional view of the sealing device according to the prior art.
A sealing device 100 serves to seal up an annular gap between an inner peripheral surface of a hydraulic cylinder 400 and an outer peripheral surface of a piston 500, and it is fitted into an annular groove 600 formed on the outer peripheral surface of the piston 500. The sealing device 100 comprises a seal ring 200 made of nylon that is in sliding contact with the inner peripheral surface of the hydraulic cylinder 400, and an elastic ring 300 that is fitted between the seal ring 200 and a groove bottom of an annular groove 600 for pushing the seal ring 200 against the inner peripheral surface of the hydraulic cylinder 400.
This sealing device 100 has an annular oil groove 700 which is formed on an outer peripheral surface of the seal ring 200, i.e., a sliding surface thereof in sliding contact with the inner peripheral surface of the hydraulic cylinder 400, along a circumferential direction thereof. This oil groove 700 functions as a pressure chamber due to the generation of accumulated pressure caused by the pressure of accumulated fluid to be sealed, whereby it serves to lubricate sliding portions and at the same time reduce an expanding force acting on the seal ring, thus making it possible to achieve a reduction in the sliding resistance of the seal ring.
However, when flaws or the like are generated in a pair of annular protrusions, respectively, which form opposite side walls of the oil groove 700, a leakage passage for the fluid to be sealed might be formed, thereby giving rise to a fear that the sealing performance of the seal ring 200 might be easily decreased. In addition, in order for the oil groove 700 to exhibit the function of the pressure chamber to a satisfactory extent, a prescribed sliding distance is required, and hence there is a possibility that an expected effect might not be achieved in an initial stage of operation.
Further, to solve these problems, there is proposed a method in which a sliding surface of a seal ring is processed by means of shot processing or the like so as to roughen the surface, thereby providing the surface with minute irregularities. However, there is the following problem. That is, as shown in FIG. 13, those portions lying between adjacent concave portions become pointed protrusions, so high surface pressure will be generated at these protrusions, whereby the minute irregularities will be worn out in a relatively early time. Here, note that FIG. 13 is a schematic cross sectional view of the irregularities formed by means of shot processing.
In addition, in case where nylon is used as a material for the piston seal, the pressure resisting ability is more excellent but the sliding resistance be comes higher, in comparison with the case of using PTFE, so there is a problem that when a hydraulic cylinder is operated, a so-called stick-slip phenomenon will occur, thus ma king it easy to generate vibration.
Moreover, as a high pressure piston seal, there has also been known a sealing device which comprises a seal ring made of PTFE, back-up rings made of nylon arranged at opposite sides of the seal ring, and a biasing ring for biasing these rings (for example, a patent document 8). However, this sealing device is composed of the four members, and hence has a problem in that it is poor from a cost point of view.    Patent Document 1: Japanese utility model application laid-open No. S60-101265    Patent Document 2: Japanese patent application laid-open No. H09-250640    Patent Document 3: Japanese utility model application laid-open No. S58-16459    Patent Document 4: Japanese patent application laid-open No. H10-213231    Patent Document 5: Japanese patent No. 3114874    Patent Document 6: Japanese utility model application laid-open No. H04-136364    Patent Document 7: Japanese utility model application laid-open No. H04-129969    Patent Document 8: Japanese utility model application laid-open No. S59-180056